Regression of #468

[Turakar]

Expected Behaviour
See #468. The pace feature should be based on the actual end slide, and not on the last slide of the PDF.

Actual Behaviour
The pace feature seems to be based on the last slide of the PDF, i.e. I can get an orange indicator on the last slide.

Steps to reproduce

1. Open demo with pdfpc and a duration, mark third slide as end slide and wait.
2. You can get an orange color on this last slide, which is unexpected.

Notes
https://github.com/pdfpc/pdfpc/blob/14344bc203cfff71a1b72deb161e003c9b480bb2/src/classes/presentation_controller.vala#L382C69-L382C82

I think that the fix in 052403d is actually wrong, it should have been metadata.get_end_user_slide() + 1.

pdfpc version: 4.6.0
Used Distribution (GTK version, Vala version): Fedora
Display Server (X11/Wayland): X11
HiDPI Setting (yes/no): unknown

[burbschat]

It seems like the actual issue might be that the fix from 052403d has been accidentally reverted in f88e1c70, which was merged into master (#634)